Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

The chemistry of isomers of icosaborane(26): Crystal and molecular structure of 1,2′-bi(nido-decaboranyl)

The crystal and molecular structure of 1,2′-(nido-B₁₀H₁₃)₂ has been determined by single-crystal X-ray diffraction analysis, thus con     

Characterization of the disordered phosphate network in CaO-P2O5 glasses by P solid-state NMR and Raman spectroscopies

In this study, the disordered network of calcium phosphate glasses is investigated by Raman scattering and ³¹P magic angle spinning (MAS) solid-state NMR spectroscopies. The use of both spectroscopies in a combined approach allows drawing a detailed understanding of the structure of these glasses. The P―O―P connectivity between successive PO₄ tetrahedra is probed using through-bond double quantum-single quantum (DQ-SQ) and triple quantum-single quantum (TQ-SQ) MAS NMR correlation experiments. Over the broad range of glass compositions studied here, two very different phosphate network topologies are encountered. The results obtained for the polyphosphate compositional range (above 50 mol% Ca) allow determining the phosphate chain-length distribution in the glass as a function of the modifier cation content. For the ultraphosphate region (below 50 mol% Ca), the network topology undergoes a sudden change close to 39 mol% Ca which can be interpreted in terms of a rigidity transition.     

All-optical measurement-based quantum-information processing in quantum dots

Parity measurements on qubits can generate the entanglement resource necessary for scalable quantum computation. Here we describe a method for fast optical parity measurements on electron spin qubits within coupled quantum dots. The measurement scheme, which can be realized with existing technology, consists of the optical excitation of excitonic states followed by monitored relaxation. Conditional on the observation of a photon, the system is projected into the odd/even-parity subspaces. Our model incorporates all the primary sources of error, including detector inefficiency, effects of spatial separation and nonresonance of the dots, and also unwanted excitations. Through an analytical treatment we establish that the scheme is robust to such effects. Two applications are presented: a realization of a controlled-NOT gate, and a technique for growing large scale graph states.     

Electronic spin precession and interferometry from spin-orbital entanglement in a double quantum dot

A double quantum dot inserted in parallel between two metallic leads can entangle the electron spin with the orbital (dot index) degree of freedom. An Aharonov-Bohm orbital phase can be transferred to the spinor wave function, providing a geometrical control of the spin precession around a fixed magnetic field. A fully coherent behavior occurs in a mixed orbital-spin Kondo regime. Evidence for the spin precession can be obtained, either using spin-polarized metallic leads or by placing the double dot in one branch of a metallic loop.     

Quantum criticality in a double-quantum-dot system

We discuss the realization of the quantum-critical non-Fermi-liquid state, originally discovered within the two-impurity Kondo model, in double-quantum-dot systems. Contrary to common belief, the corresponding fixed point is robust against particle-hole and various other asymmetries and is unstable only to charge transfer between the two dots. We propose an experimental setup where such charge transfer processes are suppressed, allowing a controlled approach to the quantum-critical state. We also discuss transport and scaling properties in the vicinity of the critical point.     

The synthesis of new weakly coordinating diborate anions: anion stability as a function of linker structure and steric bulk

The successive addition of KCN and Ph3CCl to B(C6F4-C6F5-2)3 (PBB) affords triphenylmethyl salts of the [NC-PBB]- anion. By contrast, the analogous reaction with sodium dicyanamide followed by treatment with Ph(3)CCl leads to the zwitterionic aminoborane H2NB(C12F9)2C12F8, via nucleophilic attack on an o-F atom, together with CPh3[F-PBB]. Whereas treatment of [NC-PBB]- with either PBB or B(C6F5)3 fails to give isolable cyano-bridged diborates, the reaction of Me3SiNC-B(C6F5)3 with PBB in the presence of Ph3CCl affords [Ph3C][PBB-NC-B(C6F5)3]. Due to steric hindrance this anion is prone to borane dissociation. The longer linking group N(CN)2- gives the very voluminous anions [N[CNB(C6F5)3]2]- and [N(CN-PBB)2]-. A comparison of propylene polymerisations with rac-Me2Si(Ind)2ZrMe2 activated with the various boranes or trityl borates gives an anion-dependent activity sequence, in the order [NC-PBB]- < [MeB(C6F5)3]- < [MePBB]- approximately [PBB-NCB(C6F5)3]- approximately [N[CNB(C6F5)3]2]- < [F-PBB]-< [B(C6F5)4]- < [N(CN-PBB)2]-. The anion [N(CN-PBB)2]- gives a catalyst productivity about 2500 times higher than that of [NC-PBB]- and exceeds that of [B(C6F5)4]- based catalysts. The van der Waals volumes and surface areas of the anions have been calculated and provide a rationale for the observed reactivity trends in polymerisation reactions.     

Complexation of dimethylmagnesium with alpha-diimines; structural and EPR characterisation of single electron and alkyl transfer products

Treatment of dimethylmagnesium with the alpha-diimine ligands Ar'N=C(R)C(R)=NAr' [R = naphth-1,8-diyl (1), H (2), CH3 (3); Ar' = 2,6-diisopropylphenyl] in diethyl ether provides the neutral methyl-bridged dimeric complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET) to the coordinated diimine and elimination of a methyl radical. These biradical species have been characterised by EPR spectroscopy and, for the ligand , X-ray crystallography. In the presence of THF the reaction of ligand proceeds to the diamagnetic [(ene-1,2-diamide)Mg(THF)3] complex in which the diimine ligand has been doubly reduced to an ene-diamide by two successive SET processes. Comparison of the structural data for the free ligand with that obtained for the alpha-diimine radical anion and ene-diamide complexes shows the expected increases in C-N, and decreases in C-C, bond lengths within the N-C-C-N unit consistent with the progressive reduction of the ligand. In the case of ligand , reaction at low temperature provides the complex [Mg(mu2-Me){Ar'NC(Me)2C(Me)NAr'}]2 in which methyl transfer to a ligand imine carbon atom has occurred. This species has also been structurally characterised. This contrasts with the formation of the radical species at room temperature, and indicates the involvement of an intermediate in which the radical products of the SET process are held in close proximity by the solvent cage. Two competing processes of methyl radical escape and methyl transfer to the ligand account for the formation of the observed products at different temperatures.     

Targeting large phosp(III)azane macrocyles [{P(mu-NR)}(2)(LL)](n) (n> or = 2)

The condensation reactions of the dimer [ClP(micro-NR)](2) with organic diacids [LL(H)(2)], possessing linear orientations of their organic groups, result in the formation of phospha(III)zane macrocyles of the type [{P(mu-NR)}(2)(LL)](n) of various sizes. The series of macrocycles [{P(mu-N(t)Bu)}(2){1,5-(NH)(2)C(10)H(6)}](3), [{P(mu-NCy)}(2)(1,5-O(2)C(10)H(6))](n) [n = 3; n = 4], [{P(mu-N(t)Bu)}(2){1,4-(NH)(2)C(6)H(4)}](4), [{P(mu-N(t)Bu)}(2)(1,4-O(2)C(6)H(4))], [{P(mu-NCy)}(2)(1,4-O(2)C(6)H(4))](3) and [{P(mu-N(t)Bu)}(2){(NH)C(6)H(4)OC(6)H(4)(NH)}](2) can be related to classical organic frameworks, like calixarenes.